1. Field of the Invention
The present invention relates to a method of manufacturing an optical waveguide for widespread use in the fields of optical communications, optical information processing, position sensors, and other general optics technology.
2. Description of the Related Art
In general, an optical waveguide is configured in such a manner that cores serving as a passageway for light are formed in a predetermined pattern on a surface of an under cladding layer, and that an over cladding layer is formed so as to cover the cores. In particular, for the formation of the over cladding layer of a desired shape, e.g. the over cladding layer having an end portion in the form of a lens portion, a mold including a cavity having a mold surface of a shape complementary to the desired shape of the over cladding layer is used to form the over cladding layer (as disclosed, for example, in Japanese Published Patent Application No. 2008-281654).
The assignee of the present application has proposed a mold made of a light-transmissive resin which is excellent in dimensional accuracy as a mold for the formation of the over cladding layer, and has already applied for a patent (Japanese Patent Application No. 2010-126714). This mold is produced in a manner to be described below. First, a mold member of a shape identical with the shape of the over cladding layer is prepared, and is placed in a mold production container. Then, the container is filled with a light-transmissive resin, and the light-transmissive resin is cured. The cured light-transmissive resin is taken out of the container, and the mold member is removed. This provides a mold made of a light-transmissive resin in which a hollow resulting from the removal of the mold member serves as a cavity for the formation of the over cladding layer.
An optical waveguide is produced using the above-mentioned mold in a manner to be described below. First, the cavity of the mold is filled with a photosensitive resin for the formation of the over cladding layer. Then, cores formed on a surface of an under cladding layer are immersed in the photosensitive resin, and the under cladding layer is pressed against the mold. Next, the photosensitive resin is exposed to light through the mold, whereby the photosensitive resin is cured and formed into the over cladding layer. Thereafter, the mold is removed. This provides an optical waveguide including the under cladding layer, the cores, and the over cladding layer.
In the above-mentioned method of producing a mold made of a light-transmissive resin, the reduction in the wall thickness of the mold is preferable because it results in better exposure to irradiation light (or better light transmission properties) through the mold during the formation of the over cladding layer. This wall thickness reduction, however, gives rise to a tendency toward breakage of the mold (or toward worse molding properties) during the removal of the mold member in the step of producing the mold. The increase in the wall thickness of the mold, on the other hand, reduces the tendency toward the breakage of the mold (or improves the molding properties) during the removal of the mold member, but causes worse exposure properties (or worse light transmission properties) to result in poor formation of the over cladding layer. The mold made of the light-transmissive resin still has room for improvement in this regard.
Silicone resins are used as a material for the formation of the mold. Silicone resins, however, are still soft after being cured. For this reason, when the wall thickness of a mold made of a silicone resin is reduced, the mold is more prone to breakage (or prone to worse molding properties) during the removal of the mold member. Also, a cured body of silicone resin (or a mold made of a silicone resin) has a self-adsorptive property. Thus, even when the wall thickness of the mold made of a silicone resin is increased, the mold placed on a molding stage (or a molding worktable) during the formation of the over cladding layer is prone to be deformed by elongation due to uneven contact between the mold and the molding stage, resulting in a tendency toward the occurrence of dimensional error in the mold. The occurrence of the dimensional error in the mold causes the over cladding layer to have dimensions deviated from their design values, resulting in an improper positional relationship between the cores and the over cladding layer. As a result, a light beam emitted from a distal end of each of the cores is not appropriately narrowed down by the lens portion at an end of the over cladding layer, but exits the lens portion while being widened. This decreases the intensity of received light (or light propagation characteristics) on a light-receiving side.